The Supervolcano of Sesia is a now-fossilised geographic structure which exposes some of the most hidden and deepest parts of the magma system underlying the volcano. It stretches over an area that includes, in addition to Valsesia and Valsessera, part of the western Biella area right up to lake Maggiore. One of its more appealing areas can be explored through the Prato Sesia itinerary where, by taking a pleasant stroll through the woods (about 20 minutes), you can admire the caldera rocks. Here, one of the best outcrops of volcanic megabreccia is exposed: in addition to the lava rocks (rhyolitic glass) enormous blocks of rocks are visible which were once part of the pre-existing volcano's structure, crumbling when the volcano collapsed in on itself, and included in the tuff expelled during the explosion.